Recently, FortiGuard Labs came across several malicious documents that exploit the vulnerability CVE-2012-0158. To evade suspicion from the victim, these RTF files drop decoy documents containing politically themed texts about a variety of Vietnamese government-related information. It was believed in a recent report that the hacking campaign where these documents were used was led by the Chinese hacking group 1937CN. The link to the group was found through malicious domains used as command and control servers by the attacker. In this blog, we will delve into the malware used in this campaign and will try to provide more clues as to the instigator of this campaign.

Sample decoy documents

When the documents are opened, they drop several files in one of the following folders:

%AppData%\Microsoft\Credentials

%AppData%\Microsoft\SystemCertificates

%AppData%\Microsoft\Windows\Templates

Some samples drop the following files:

Taskeng.exe – signed legitimate GoogleUpdate.exe version 1.3.33.5

Psisrndrx.ebd – encrypted blob containing malware file

Goopdate.dll – decrypter and loader of malware file

Some drop the following files:

SC&Cfg.exe – signed legitimate McAfee AV application

Vsodscpl.dll – contains the malware file

Others drop the following files:

Systemm.exe - signed legitimate GoogleUpdate.exe version 1.3.30.3

Systemsfb.ebd - encrypted blob containing malware file

Goopdate.dll – decrypter and loader of malware file

Similar to other APT attacks, such as MONSOON APT, this APT uses DLL hijacking to evade the behavior monitoring technologies of security programs.

DLL Hijacking

DLL hijacking is a technique used by some APT malware in which instead of the legitimate application (.exe) loading the benign DLL, the application is tricked into loading a DLL containing malicious code. This technique is employed to evade Host Intrusion Prevention System (HIPS) of security programs that monitor the behaviors of executed files. Most HIPS tools whitelist signed or trusted files, thereby excluding malware loaded using DLL hijacking by signed files from behavior monitoring.

In the context of this attack, taskeng.exe and SC&Cfg.exe are signed legitimate applications; however, they are tricked into loading malware that are disguised as the legitimate Goopdate.dll and Vsodscpl.dll files.

Taskeng.exe and SC&Cfg.exe file information

Next, Taskeng.exe needs to load and import some functions from the original Goopdate.dll file; however, the Goopdate.dll was hijacked to contain malicious code, effectively changing the original code execution to execution of the malicious code.

Snippet from taskenge.exe that loads goopdate.dll

In the same fashion, SC&Cfg.exe imports the “dll_wWinMain” function from the original vsodscpl.dll, but this DLL was hijacked as well, and also contains malicious code.

SC&Cfg.exe import table containing import from vsodscpl.dll

Once the malicious DLLs are loaded, the DLLs decrypt (from psisrndrx.ebd (1st case) or from its body (2nd case)) and load a Trojan downloader. The Trojan downloader is a DLL file. It is not dropped on disk but is only executed in memory. Also, the actual Trojan downloader in memory when dumped will not run. This is because the ‘MZ’ in the IMAGE_DOS_HEADER, the DOS stub, and the ‘PE’ signature were deliberately removed. This was done to prevent the dumped file from being analyzed properly in a debugger and decompiler. However, we can easily fix the dump by adding the ‘MZ’, a DOS stub, and the ‘PE’ signature.

Missing header items as anti-analysis

This Trojan downloader downloads a RAT (Remote Access Trojan), which we will call “NewCore” RAT, from the following domains:

web.thoitietvietnam.org

dalat.dulichovietnam.net

halong.dulichculao.com

Trojan Downloader

The Trojan downloader first creates an autostart registry entry so it runs whenever the machine is rebooted:

HKLM/HKCU\Software\Microsoft\Windows\CurrentVersion\Run

Microsoft Windows Media = “%AppData%\Microsoft\Credentials\.exe”

As an anti-VM, it checks whether the environment has the registry key:

HKCR\Applications\VMwareHostOpen.exe

Before it can download the NewCore RAT, it needs to send the following information to the C&C server:

OS version

Processor speed

Number of processors

Physical memory size

Computer name

User name

User privilege

Computer IP address

Volume serial number

The above information is converted to its hex string representation, and then sent to the C&C server via HTTP GET:

GET request to the C&C server

The response is an XOR encrypted data that includes the encrypted NewCore RAT.

Response from the server

XOR decryption of response from the C&C server

NewCore Remote Access Trojan

We named this RAT ‘NewCore’ after we found the project name used by the author, which is indicated on the following PDB file string:

Malware project name

According to its compilation time stamp, this malware was compiled on March 16, 2017.

Compilation time

However, as of this writing, only a few Antivirus engines, including Fortinet detect this malware according to VirusTotal.

VirusTotal positives

This RAT is a DLL file. Its malicious routines are contained in its imported function “ProcessTrans”. However, executing the DLL without using the downloader will not work as the C&C server string is not embedded in its body. When the downloader calls the function “ProcessTrans”, it supplies to the function the C&C server string and a handle to the C&C server internet session. In this case, Heuristic detection based on behavior will not work on the DLL alone.

This RAT is capable of the following:

Shutdown the machine

Restart the machine

Get disk list

Get directory list

Get file information

Get disk information

Rename files

Copy files

Delete files

Execute files

Search files

Download files

Upload files

Screen monitoring

Start command shell

Shutdown and restart machine commands

File manager, monitor screen, command shell commands

File manager subcommands

Based on the strings found in its body, this malware may have been derived from the PcClient and PcCortr backdoors whose source codes are publicly available, especially on Chinese language coding forums. PcClient detections usually include PcCortr.

Strings related to PcCortr modules

PcClient and PcCortr source codes can be downloaded from Chinese coding forums

PcClient was used in the past by some APT groups such as Nitro, which were also linked to a China-based hacker.

According to the PDB file string embedded in the NewCore RAT body, the creator of the project is someone using the handle “hoogle168”.

We have little clue as to who this individual is, so we tried to look for information about this handle. Our investigation led us to several Chinese language forum pages. Looking at these forums, it seems like a user using the handle “hoogle168” is very active on a certain coding forums, and is proficient in C and VC++. This user even replied to a thread and gave advice on what to learn to develop remote control software. We don’t know for sure if this person is the NewCore author.

Solution:

To prevent triggering this RTF exploit, it is important to apply the patches released by Microsoft that cover CVE-2012-0158 vulnerability.

Fortinet also covers detection for these threats as MSOffice/Dropper!exploit.CVE20120158 for the malicious RTF files, and W32/NewCore.A!tr.bdr for the payload.

Conclusion:

NewCore RAT may just be a rehashed PcClient RAT, but it proves to be effective in evading AV detection by using a combination of simple techniques such as DLL-hijacking, file-less execution of downloaded malware, and passing C&C information as parameter from downloader to the downloaded file.

As always, Fortiguard Labs will keep an eye on threats like NewCore to protect our customers against these threats.

Thank you to Tien Phan for additional insights.

-= FortiGuard Lion Team =-

IOCs:

Lure:

2a4e8ae006be3a5ed2327b6422c4c6f8f274cfa9385c4a540bc617bff6a0f060

3faacef20002f9deb1305c43ea75b8422fd29a1559c0cf01cf1cee6a1b94fc0e

5bdbf536e12c9150d15ae4af2d825ff2ec432d5147b0c3404c5d24655d9ebe52

14b4d8f787d11c7d72f66231e80997ef6ffa1d868d9d8f964bea36871e1c2ff2

637c156508949c881763c019d2dca7c912da9ec63f01e3d3ba604f31b36e52ab

5573f6ec22026b0c00945eec177f04212492bb05c33b4b80f73c65ce7fe5119a

00466938836129a634b573d2b57311200ab04aba7252cfbf6b77f435612ca6c6

c375946ba8abee48948f79a89ea5b4f823d8287c2feb3515755b22ba5bd8849d

f6a4bab7d5664d7802f1007daa04ae71e0e2b829cd06faa9b93a465546837eb4

fabf4debacb7950d403a84f4af25c084d0b576783006d334052ebf7ea432196e

Loader:

9cebae97a067cd7c2be50d7fd8afe5e9cf935c11914a1ab5ff59e91c1e7e5fc4

ea5b3320c5bbe2331fa3c0bd0adb3ec91f0aed97709e1b869b79f6a604ba002f

Trojan Downloader:

edbcc384b8ae0a2f52f239e2e599c3d2053f98cc1f4bc91548ec420bec063be6

49efab1dedc6fffe5a8f980688a5ebefce1be3d0d180d5dd035f02ce396c9966

df8475669a14a335c46c802f642dd5569c52f915093a680175c30cc9f28aacdb

NewCore RAT:

37bd97779e854ea2fc43486ddb831a5acfd19cf89f06823c9fd3b20134cb1c35

Command and Control Servers:

web.thoitietvietnam.org

dalat.dulichovietnam.net

halong.dulichculao.com

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